1/*-
2 * Copyright (c) 2009, 2010 Xin LI <delphij@FreeBSD.org>
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 *    notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 *    notice, this list of conditions and the following disclaimer in the
12 *    documentation and/or other materials provided with the distribution.
13 *
14 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
15 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
16 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
17 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
18 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
19 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
20 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
21 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
22 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
23 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
24 * SUCH DAMAGE.
25 */
26
27#include <sys/cdefs.h>
28__FBSDID("$FreeBSD$");
29
30#include <sys/libkern.h>
31#include <sys/limits.h>
32
33/*
34 * Portable strlen() for 32-bit and 64-bit systems.
35 *
36 * Rationale: it is generally much more efficient to do word length
37 * operations and avoid branches on modern computer systems, as
38 * compared to byte-length operations with a lot of branches.
39 *
40 * The expression:
41 *
42 *	((x - 0x01....01) & ~x & 0x80....80)
43 *
44 * would evaluate to a non-zero value iff any of the bytes in the
45 * original word is zero.
46 *
47 * On multi-issue processors, we can divide the above expression into:
48 *	a)  (x - 0x01....01)
49 *	b) (~x & 0x80....80)
50 *	c) a & b
51 *
52 * Where, a) and b) can be partially computed in parallel.
53 *
54 * The algorithm above is found on "Hacker's Delight" by
55 * Henry S. Warren, Jr.
56 */
57
58/* Magic numbers for the algorithm */
59#if LONG_BIT == 32
60static const unsigned long mask01 = 0x01010101;
61static const unsigned long mask80 = 0x80808080;
62#elif LONG_BIT == 64
63static const unsigned long mask01 = 0x0101010101010101;
64static const unsigned long mask80 = 0x8080808080808080;
65#else
66#error Unsupported word size
67#endif
68
69#define	LONGPTR_MASK (sizeof(long) - 1)
70
71/*
72 * Helper macro to return string length if we caught the zero
73 * byte.
74 */
75#define testbyte(x)				\
76	do {					\
77		if (p[x] == '\0')		\
78		    return (p - str + x);	\
79	} while (0)
80
81size_t
82strlen(const char *str)
83{
84	const char *p;
85	const unsigned long *lp;
86	long va, vb;
87
88	/*
89	 * Before trying the hard (unaligned byte-by-byte access) way
90	 * to figure out whether there is a nul character, try to see
91	 * if there is a nul character is within this accessible word
92	 * first.
93	 *
94	 * p and (p & ~LONGPTR_MASK) must be equally accessible since
95	 * they always fall in the same memory page, as long as page
96	 * boundaries is integral multiple of word size.
97	 */
98	lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK);
99	va = (*lp - mask01);
100	vb = ((~*lp) & mask80);
101	lp++;
102	if (va & vb)
103		/* Check if we have \0 in the first part */
104		for (p = str; p < (const char *)lp; p++)
105			if (*p == '\0')
106				return (p - str);
107
108	/* Scan the rest of the string using word sized operation */
109	for (; ; lp++) {
110		va = (*lp - mask01);
111		vb = ((~*lp) & mask80);
112		if (va & vb) {
113			p = (const char *)(lp);
114			testbyte(0);
115			testbyte(1);
116			testbyte(2);
117			testbyte(3);
118#if (LONG_BIT >= 64)
119			testbyte(4);
120			testbyte(5);
121			testbyte(6);
122			testbyte(7);
123#endif
124		}
125	}
126
127	/* NOTREACHED */
128	return (0);
129}
130